oysteikt
/
mpd
0
0
Fork 0
Code Issues Pull Requests Releases Activity

decoder/HybridDSD: add code comments

This commit is contained in:
Max Kellermann 2018-02-13 09:29:58 +01:00
parent b2ec5d0f01
commit 58bb866e2d
2 changed files with 34 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
doc/user.xml
View File

@ -2766,12 +2766,16 @@ run</programlisting>
<title><varname>hybrid_dsd</varname></title>
<para>
Hybrid-DSD is a MP4 container file
(<filename>*.m4a</filename>) which contains both ALAC and
DSD data. It is disabled by default, and works only if you
explicitly enable it. Without this plugin, the ALAC parts
gets handled by the <link linkend="ffmpeg_decoder">FFmpeg
decoder plugin</link>.
<ulink
url="http://dsdmaster.blogspot.de/p/bitperfect-introduces-hybrid-dsd-file.html">Hybrid-DSD</ulink>
is a MP4 container file (<filename>*.m4a</filename>) which
contains both ALAC and DSD data. It is disabled by default,
and works only if you explicitly enable it. Without this
plugin, the ALAC parts gets handled by the <link
linkend="ffmpeg_decoder">FFmpeg decoder plugin</link>. This
plugin should be enabled only if you have a bit-perfect
playback path to a DSD-capable DAC; for everybody else,
playing back the ALAC copy of the file is better.
</para>
</section>

28
src/decoder/plugins/HybridDsdDecoderPlugin.cxx
View File

@ -36,6 +36,9 @@ namespace {
static bool
InitHybridDsdDecoder(const ConfigBlock &block)
{
/* this plugin is disabled by default because for people
without a DSD DAC, the PCM (=ALAC) part of the file is
better */
if (block.GetBlockParam("enabled") == nullptr) {
LogInfo(hybrid_dsd_domain,
"The Hybrid DSD decoder is disabled because it was not explicitly enabled");
@ -45,6 +48,10 @@ InitHybridDsdDecoder(const ConfigBlock &block)
return true;
}
/**
* This exception gets thrown by FindHybridDsdData() to indicate a
* file format error.
*/
struct UnsupportedFile {};
struct Mp4ChunkHeader {
@ -98,6 +105,7 @@ static std::pair<AudioFormat, offset_type>
FindHybridDsdData(DecoderClient &client, InputStream &input)
{
auto audio_format = AudioFormat::Undefined();
bool found_version = false;
while (true) {
auto header = ReadHeader(client, input);
@ -107,12 +115,13 @@ FindHybridDsdData(DecoderClient &client, InputStream &input)
size_t remaining = header_size - sizeof(header);
if (memcmp(header.type, "bphv", 4) == 0) {
/* ? */
/* version; this plugin knows only version
1 */
if (remaining != 4 || ReadBE32(client, input) != 1)
throw UnsupportedFile();
remaining -= 4;
audio_format.format = SampleFormat::DSD;
found_version = true;
} else if (memcmp(header.type, "bphc", 4) == 0) {
/* channel count */
if (remaining != 4)
@ -139,18 +148,23 @@ FindHybridDsdData(DecoderClient &client, InputStream &input)
audio_format.sample_rate = sample_rate;
} else if (memcmp(header.type, "bphf", 4) == 0) {
/* ? */
/* format: 0 = plain DSD; 1 = DST compressed
(only plain DSD is understood by this
plugin) */
if (remaining != 4 || ReadBE32(client, input) != 0)
throw UnsupportedFile();
remaining -= 4;
audio_format.format = SampleFormat::DSD;
} else if (memcmp(header.type, "bphd", 4) == 0) {
/* the actual DSD data */
if (!audio_format.IsValid())
if (!found_version || !audio_format.IsValid())
throw UnsupportedFile();
return std::make_pair(audio_format, remaining);
}
/* skip this chunk payload */
input.LockSkip(remaining);
}
}
@ -175,6 +189,8 @@ HybridDsdDecode(DecoderClient &client, InputStream &input)
frame_size = result.first.GetFrameSize();
remaining_bytes = result.second;
} catch (UnsupportedFile) {
/* not a Hybrid-DSD file; let the next decoder plugin
(e.g. FFmpeg) handle it */
return;
}
@ -195,6 +211,7 @@ HybridDsdDecode(DecoderClient &client, InputStream &input)
break;
}
/* fill the buffer */
auto w = buffer.Write();
if (!w.empty()) {
if (remaining_bytes < (1<<30ull) &&
@ -210,6 +227,7 @@ HybridDsdDecode(DecoderClient &client, InputStream &input)
buffer.Append(nbytes);
}
/* submit the buffer to our client */
auto r = buffer.Read();
auto n_frames = r.size / frame_size;
if (n_frames > 0) {
@ -233,6 +251,8 @@ const struct DecoderPlugin hybrid_dsd_decoder_plugin = {
HybridDsdDecode,
nullptr,
nullptr,
/* no scan method here; the FFmpeg plugin will do that for us,
and we only do the decoding */
nullptr,
nullptr,
hybrid_dsd_suffixes,